It is known to use power steering in road vehicles, e.g., electrical power assisted steering, commonly abbreviated as EPAS, in a road vehicle such as a car, lorry, bus or truck, wherein an electric motor assists a driver of the road vehicle by adding an assistive torque to e.g., a steering column of the road vehicle.
It is further known to use semi-autonomous steering systems, such as lane keeping aid systems, in order to help a road vehicle driver maintain the road vehicle in a desired lane. For lane keeping aid systems where an EPAS is used, a steering wheel torque overlay, i.e., additional steering wheel torque on top of what would have been obtained by the base assist of the EPAS, is used for lateral position control.
Document EP 2 591 983 A1 discloses a method and a system for adaptation of a steering wheel torque overlay of a lane keeping aid system. The method provides logic to override overlay torque from a semi-autonomous steering function, for instance a lane keeping aid, both in general but also in specific scenarios like driving in curves. Scaling of an overlay torque is applied, using a scaling factor that is a function of driver applied steering wheel torque. In the method of EP 2 591 983 A1 the driver applied steering wheel torque directly influences a calculated scaling factor.
However, there are situations where a driver for some reason and during a significant period of time could choose not to follow steering guidance from a position controller of a semi-autonomous steering function, such as that of EP 2 591 983 A1, and instead continuously try to resist or override the overlay torque applied. The effect of such situations is an interaction between the overlay torque and the torque applied by the driver, potentially causing fluctuations in the calculated scaling factor, and because of that, also clearly noticeable torque artifacts from the semi-autonomous steering system.